For example, at the time of evaluation of a combustion state in a combustion chamber of an internal combustion engine of a car, measurement is performed using an optical measurement probe for guiding light that is generated at the time of combustion to an appliance (for example, see Patent Document 1). This type of optical measurement probe is provided with a light guide made of optical fiber, for example, and is capable of causing light in a predetermined field of view range to enter the light guide, and of guiding the light to an appliance by the light guide.
FIG. 6 is a cross-sectional diagram showing main sections of an example structure of a conventional optical measurement probe 101. The optical measurement probe 101 includes an optical window 102 for transmitting incident light, and a main body 103 holding the optical window 102. In this example, light entering the main body 103 from the optical window 102 is detected by a detector (not shown) through an optical fiber 104.
The main body 103 is a cylindrical member, for example, and the optical window 102 is accommodated at one end section. Specifically, a recessed section having an inner diameter corresponding to the outer diameter of the optical window 102 is formed to the one end section of the main body 103, and the recessed section forms an optical window accommodation section 131 for accommodating the optical window 102. The optical window 102 is formed as a columnar shape, for example.
The optical window 102 is provided adjacent to the optical fiber 104, on a same axial line L100. An end surface of the optical window 102, on the opposite side of the optical fiber 104, forms an incident surface 121 into which light enters from outside. light entering from the incident surface 121 is transmitted inside the optical window 102 along the axial line L100, and is guided to the optical fiber 104 side.
Of the light entering from the incident surface 121, only the light in a predetermined field of view range R100 with the axial line L100 at the center enters the optical fiber 104. The field of view range R100 is dependent on the number of apertures (NA) of the optical fiber 104 and the shape of the optical window 102.